In a system in which a transmitter shares an antenna with a receiver, a signal from the transmitter is input to the receiver, so that the receiver may be saturated or may be broken down. A radio-frequency identification (RFID) transceiver can be taken as an example of the aforementioned system.
In the past, in order to remove the problem, it was common, as shown in FIG. 1, to use an isolator 400 or a circulator between a receiver 100, a transmitter 200 and an antenna 300.
However, the isolator 400 or the circulator and so on is difficult to be integrated as a system on a chip (SoC) and requires external elements. Therefore, a form factor and a total system area are increased, so that a production cost rises.
Moreover, even though the isolator 400 is used, the incomplete performance of the isolator 400 may cause a signal of the transmitter 200 to be still leaked to the receiver 100 of the same transceiver. This is indicated by a leakage signal 1 in FIG. 1. Due to the reflection property of the antenna 300, a portion of the transmitter 200 signal which has been transferred to the antenna 300 may flow into the receiver 100. This is indicated by a leakage signal 2 in FIG. 1.
Here, the magnitudes of the leakage signals 1 and 2 introduced into the receiver 100 are much larger than an external signal that should be received by the receiver 100 through the antenna 300. As a result, the receiver 100 requires a large dynamic range.
The leakage signals 1 and 2 from the transmitter 200 to the receiver 100 not only increase the dynamic range required by the receiver 100 but also generate noise to the received signal of the receiver 100. Here, the noise is affected by a phase noise of a local oscillator (LO) as well as the performance of the isolator 400 and so on.
φN(t) is defined as a phase noise characteristic of a local oscillator signal of the transmitter 200 at the time of generating an output signal of the transmitter 200. Here, at a point of time when the output signal of the transmitter 200 is introduced into the receiver 100 and coupled to a receiver 100's local oscillator signal which is input to a mixer 110, a phase noise characteristic of the local oscillator signal of the receiver 100 is indicated by φN′(t) (=φN(t−τ)). “τ” represents a time required for the leakage signal of the transmitter 200 to reach the mixer 110 of the receiver 100. This is shown in FIG. 2. Therefore, even when the transmitter 200 and the receiver 100 of a transceiver system use a common local oscillator 500, it can be found that there is a difference between the phase noise characteristic of the leakage signal of the transmitter 200 and the phase noise characteristic of the local oscillator signal of the receiver 100. Hereafter, the noise is referred to as PN-induced noise. The more the “τ” increases, the more the difference between the φN′(t) and φN(t) increases. Accordingly, the more the “τ” increases, the more the PN-induced noise and receiver noise increase.
In FIG. 2, VN represents the baseband noise of the receiver 100 and can be expressed as below.
                              V          N                ≃                              α            2                    ·                      (                                          cos                ⁢                                                                  ⁢                θ                            +                                                                    (                                                                  ϕ                        N                        ′                                            -                                              ϕ                        N                                                              )                                    ·                  sin                                ⁢                                                                  ⁢                θ                                      )                                              equation        ⁢                                  ⁢                  (          1          )                    
Here, the first term including cos θ represents a DC offset. The second term including sin θ represents the PN-induced noise. “θ” relates to “τ” and represents a phase delay from when the transmitter generates a signal to when either the leakage signal 1 or the leakage signal 2 is input to the mixer 110 of the receiver. Here, since the DC offset and the PN-induced noise are represented by sine and cosine respectively, it is not easy to minimize both of them at the same time. Though an external element such as the isolator 400 can be used to reduce the magnitude itself of the leakage signal, that is, “α”, there is a limit to use the external element as described above.
Accordingly, it is necessary to provide a transceiver which reduces the noise of the receiver and improves its performance without using external devices. Moreover, receive sensitivity is required to be improved.